1. Field of the Invention
The present invention relates to an electrophotographic photoconductor for use in electrophotographic image forming apparatuses such as copying machines, facsimile machines, laser-beam printers and direct digital (computer-to-plate) plate making machines. The present invention also relates to a method for preparing the photoconductor, and an image forming apparatus and a process cartridge using the electrophotographic photoconductor.
2. Description of the Related Art
Photoconductive materials for use in electrophotographic apparatuses such as copying machines and laser-beam printers have changed from inorganic photoconductive materials such as selenium, zinc oxide and cadmium sulfide to organic photoconductive materials (OPCs). This is because organic photoconductive materials are friendly to environment and have low manufacturing costs and good designing flexibility.
Organic photoconductors are broadly classified, for example, as the following three types: (1) homogeneous single-layered photoconductors in which a layer of a photoconductive resin such as polyvinylcarbazole (PVK) or a charge transfer complex such as PVK-TNF (2,4,7-trinitrofluorenone) is arranged over an electroconductive substrate; (2) dispersion type single-layered photoconductors in which a resin layer including a pigment such as phthalocyanine or perylene dispersed in the resin is arranged over an electroconductive substrate; and (3) functionally-separated multi-layered photoconductors in which a charge generation layer (hereinafter referred to as a CGL) including a charge generating material such as an azo pigment, and a charge transport layer (hereinafter referred to as a CTL) including a charge transporting material such as triphenylamine are arranged over an electroconductive substrate.
The functionally-separated multi-layered photoconductors typically have a structure in which a charge transport layer is arranged over a charge generation layer. Functionally-separated multi-layered photoconductors having a reverse structure are sometimes referred to as reverse-layered photoconductors. In particular, the functionally-separated multi-layered photoconductors have high photosensitivity and good flexibility for designing photoconductors having high photosensitivity and good durability. The multi-layered photoconductors are therefore widely used as organic photoconductors.
In recent years, products must be manufactured while considering influence thereof on the global environment. Therefore, photoconductors for use in electrophotographic image forming apparatuses must be one of mechanical parts instead of a supply (i.e., a disposable product). In other words, photoconductors must have a long life through all the processes from manufacture of raw materials, transportation of products and treatment of wasted products. Therefore, recent photoconductors must be avoided from abrasion and scratching from the viewpoints of designing and usability of the photoconductors. In addition, photoconductor contact-carrying members around photoconductors must be avoided from damaging. Thus, image-forming engines can be avoided from degradation with time, and the exchange frequency of parts and replacement purchases of the apparatuses are reduced, which contributes to source saving, air pollution control and reduction of environmental burdens.
Some toners for use in electrophotographic apparatuses often invite a filming phenomenon on the surface of the photoconductor. Images outputted while constitutional materials of toners form such a film on the surface of the photoconductor often become deformed images with unclear outlines. The filming phenomenon must be avoided in order to use photoconductors over a long time. In some electrophotographic processes using spherical toners, the function of cleaning residual toners is derived from the lubricating property of a surface of the photoconductor. In these cases, photoconductors must have coefficients of surface friction at specific levels.
To improve abrasion resistance of organic photoconductors, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-288113 discloses a technique of copolymerizing a charge transporting material in a thermoplastic resin such as a polycarbonate.
This technique can reduce the content of a low-molecular compound to be incorporated in a surface top layer. Most of such low-molecular compounds to be used in surface top layers of photoconductors serve as antiplasticizers. Reducing the content of the low-molecular compounds enables the resin to exhibit inherent flexibility thereof The resulting resin has an increased resistance against mechanical stress and can exhibit high charge transport ability. The photoconductor can thereby stably exhibit satisfactory electrostatic performance.
The photoconductor prepared according to this technique, however, has a high production cost due to copolymerization of materials and is not practical.
Alternatively, JP-A No. 2002-229227, for example, discloses a technique of separating the functions of a charge transport layer as multiple layers and incorporating a highly hard filler into an upper (surface) charge transport layer.
This technique can impart mechanical durability and resistance against electrical stress to organic photoconductors at relatively low costs.
Some of the resulting photoconductors prepared according to this technique, however, work insufficiently in a cleaning process in electrophotographic processes. In addition, when the highly hard filler is contained in a high content or the charge transport layer to which the highly hard filler is incorporated is thickened, the photoconductors have deteriorated sensitivities. Thus, electrophotographic apparatuses may not be designed flexibly.
In contrast to such organic photoconductors, amorphous silicon photoconductors classified as inorganic photoconductors show very satisfactory mechanical strength.
The amorphous silicon photoconductors, however, have a low dielectric constant and thereby have low charge ability. In addition, they often require the use of drum heaters for suppressing image blur, thus increase power consumption of electrophotographic apparatuses and require relatively high production cost. Thus, electrophotographic apparatuses using the amorphous silicon photoconductors are generally of high cost and are commercially available in only limited applications. Most of accumulated techniques for prolonging lives of photoconductors under consideration for reduction of environmental burdens are individually useful. These conventional techniques, however, have considerable defects and are insufficient as measures for environmental issues which increasingly receive attention.